Two-dimensional (2D) material-based P–N junctions are unique building blocks for next-generation optoelectronics with the advantages of atomic thinness, flexibility and low cost. However, their device performances are limited due to either weak photo-absorption or low-quality junctions. Herein, we demonstrate a high-quality homogeneous and seamless P–N junction based on multilayer WSe₂ instead of the reported monolayer structure or van der Waals junction and present its optoelectronic applications as a diode, photovoltaic cell and photodetector with high performances. Our WSe₂ P–N junction exhibits ideal diode behaviour with an ideal factor close to 1 and rectifying ratio of 10⁵. It also shows significant photovoltaic properties with V_OC of ∼0.8 V and PCE of up to 6.2% under 635 nm light illumination. As a photodiode, it produces the maximum responsivity of 486 mA W⁻¹, external quantum efficiency of ∼90%, fast speed below 4 ms and broadband spectral coverage up to the near-infrared region. This study demonstrates a high-performance 2D-based P–N diode as multifunctional optoelectronics in a single device, which holds promise to revolutionize electronic and optoelectronic technology.